The social network announced today that it's expanding access to its custom audience tool -- a tool that lets companies target their ads to existing customers based on their phone numbers and e-mail addresses. The tool, one of Facebook's several advertising tools that rely on user data, rolled out in September. The contact information is not directly shared between companies -- it's hashed to protect privacy and then used to place ads, according to Facebook.

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Facebook will now let the third-party companies that manage these contact lists for brands access the tool. The companies are Epsilon, Acxiom, BlueKa, and Datalogix. Datalogix is already an existing Facebook partner. The companies teamed up last year to find out which ads lead to purchases, and the move caused concern among privacy advocates.

Here's Facebook's example of how the third-party companies, which already have access to lots of customer information, can use the tool:

For example, an auto dealer may want to customize an offer to people who are looking to buy a new car. To do this today, many businesses work with third parties to better understand how to identify and reach that audience. With today's updates businesses can now do this same thing by showing ads to people on Facebook who may be in the market for a new car.

This means advertisers will have more information to work with to, in theory, show users more relevant ads, and make more money for Facebook.